This invention relates to a process for loading a carrier with a catalytically active composition or with a precursor for a catalytically active composition, as well as to a catalyst produced by said process.
In solid catalysts, the catalytic active species is generally bound to the surface of the solid. For this reason the area of the active surface and its accessibility to reactants are co-determinative of the activity and selectivity of the catalyst. Activity, as used in this context, means the conversion of the reactants per unit volume of the catalyst and selectivity means the degree in which a desired reaction is accelerated relative to the acceleration of undesirable reactions. According to this definition of the activity, the area of the catalytically active surface per unit volume of the catalyst is determinative.
A high activity, therefore, requires a large active area per unit volume, which can only be achieved with small particles of the active component. The need of using catalytically active components in finely-divided form has a number of major drawbacks.
First, solid catalysts must generally have a high mechanical strength. It is difficult, however, to process finely-divided material to mechanically strong moldings while retaining a high porosity, because the pore structure thus obtained is generally unfavorable. In fact, the pores are often too narrow pores with too large effective pore length ("tortuousness"). Moreover, in the case of many catalytically active elements and compounds, processing to mechanically strong moldings is not possible at all.
Second, many catalytic reactions are carried out at temperatures and in gas atmospheres whereby small particles of the active component are rapidly sintered to form much larger particles; the concomitant decrease in active area leads to a rapid loss in activity.
Third, as the pores in a system of very small particles are mostly narrow, the transport of reactants and reaction products in the catalyst is difficult. This leads to a reduced activity of the catalyst, while its selectivity is also adversely affected.
To overcome the problems outlined above, so-called carriers or supports are generally used in heterogeneous catalysts. A carrier can be satisfactorily processed to a thermostable, mechanically strong body of the desired dimensions, while the pore structure of the carrier can be effectively adjusted. The catalytically active composition, or a precursor thereof, is applied to the carrier surface, mostly in extremely finely-divided form. Generally speaking, the surface of the carrier is not catalytically active. Although the carrier greatly dilutes the catalytically active composition, the fact that the catalytically active composition is no longer sintered at high temperatures causes the thermostable catalytically active surface area to be considerably greater than without the use of a carrier.
To promote as efficient a utilization of the carrier as possible, the active component must be applied to the surface of the carrier in finely-divided form and as homogeneously as possible. In principle, it is possible to load small particles of the desired carrier relatively simply with the catalytically active composition or a precursor thereof in such a manner that it is uniformly distributed over the carrier surface as small particles. Precursor as used in this context means the element or compound which later is converted into the catalytically active composition, for example, by a thermal treatment or a chemical reaction such as reduction.
In the preparation of solid catalysts on a technical scale, the application of the catalytically active composition to the carrier is in many cases problematic. In fact, the active component cannot in many instances be applied to the carrier. For example, in the case of many catalytically active, zero valent metals, it is impossible for them to be directly applied to a carrier in finely-divided form. In such cases, a hydrated oxide of the metal, the catalytic precursor, is applied to a carrier in finely-divided form. Thereafter, during a separate thermal pre-treatment, the metal oxide is reduced to the metal.
When small particles of the carrier are used, the pores present in the carrier are short. As a result, the migration of the precursor of the active component into the carrier particle can readily proceed. However, small particles of carrier cannot be used in some much-used catalytic reactors, namely, those in which the catalyst is present in a solid bed. When small particles of carriers are used in a solid bed reactor, the pressure drop across the catalyst bed becomes intolerably high. Also, "channeling" often occurs, which is the phenomenon that the reactants flow through a limited part of the cross-section of the reactor only, namely, at the places where the particles are in motion.
A necessary characteristic of useful catalysts is a high mechanical strength. During the filling of a reactor, substantial forces are exerted on the catalyst. During the starting-up and stopping of the reactor, the catalyst is often subjected to large differences in temperature. Breakage of catalyst particles or bodies in the reactor is highly undesirable. This leads to poor distribution of the reactants over the cross-section of the catalyst bed or over a number of parallel-connected reactor tubes.
In some, but not all, cases, it is possible to load a powdered carrier first with the catalytically active composition or a precursor, for example, as described above--and subsequently to process the composite to larger moldings having the necessary mechanical strength.
Many catalyst systems which, by themselves, are attractive, have never been applied on a technical scale, because it was impossible for them to be processed to bodies having the required mechanical strength. For this reason there has been a need for a long time of a technically feasible method of loading carrier bodies or particles with the active composition or precursor. In fact, using this, first carrier bodies of the desired sizes and mechanical strength can be made. Thereafter, these bodies are loaded with the active composition or precursor thereof.
The methods used according to the state of the art for applying the active compositions or precursors to carrier bodies do not usually lead to the desired uniform distribution of the active composition. The procedure most commonly used is the impregnation of the carrier particle bodies with a solution of a precursor of the active composition, the solvent of which is removed by drying. However, it is often observed that the precursor of the active composition is only deposited on the outer surface of the carrier bodies or at the pore mouths.
Some authors believe that viscous flow owing to capillary forces is the driving force behind the migration to the outside of the carrier body of the nonvolatile components of the solution remaining behind during drying (N. Kotter and L. Riekert in "Preparation of Catalysts II", B. Delmon, P. Grange, P. Jacobs and G. Poncelet, eds., pp. 51-63, Elsevier, Amsterdam, 1979).
These authors have therefore proposed to use a viscous solution of the active composition in the impregnation. As the authors state, the use of a viscous solution does indeed lead to a somewhat more homogeneous distribution of the active composition on the carrier. One disadvantage of impregnating with a viscous solution is, however, that the viscous solution cannot well penetrate into long, relatively narrow pores, as generally occur in shaped carrier bodies or particles.
Other authors believe that the gas which remains behind in the pores of the carrier body during the impregnation forces the nonvolatile elements of the solution to the outside of the carrier particles (S. Y. Lee and R. Aris in "Preparation of Catalysts III", P. Poncelet, P. Grange and P. A. Jacobs, eds., pp. 35-45, Elsevier, Amsterdam, 1983). As can be inferred from the detailed publication by Lee and Aris, many factors which are difficult to control play a role in the impregnation and drying of catalyst bodies such as pellets. The result is that the distribution of the active component after impregnation and drying is often far from uniform.
As observed above, this is generally unfavorable for the activity of the catalyst. In some cases, where poisoning of the active composition occurs, the active composition is deliberately distributed non-uniformly over the carrier body. This practice is generally unattractive.
In order to render the distribution more homogeneous, the above-cited teaching of Kotter and Riekert proposed to increase the viscosity of the solution used for impregnation by adding hydroxyethyl cellulose to the solution. While this results in a homogeneous distribution of the precursor, unfortunately, the viscous solution of the precursor can hardly, if at all, penetrate the narrow pores of a carrier body. Thus, no homogeneous distribution is obtained during the impregnation.
According to another known method, the so-called deposition-precipitation technique, an amount of solution of the active component is added which is just sufficient to fill the pores of the carrier body. This method is also sometimes referred to as an embodiment of dry impregnation or "incipient wetness" impregnation. In addition to the catalytically active composition, the solution contains urea, ammonium cyanate or another compound which upon hydrolysis increases the pH. After the bodies have been impregnated at such a low temperature that there is no appreciable hydrolysis, the temperature is increased. The active composition, including precursor thereof, can then deposit on the surface of the carrier, provided there is a sufficient interaction between the nuclei of the precipitating composition and the carrier surface. In some cases, however, this process cannot be used. In some cases, there is no good interaction with the carrier. In others, there are a number of important active materials which cannot be precipitated by increasing the pH of the liquid.
There is another condition which should be satisfied in developing a universal method of applying a catalytically active composition, including precursor thereof, to carrier bodies, namely, that the accessible area and the pore distribution of the resulting laden carrier should not differ from that of the non-laden carrier. Thus, in particular to obtain a good selectivity, the pore structure of the ultimate catalyst must be capable of being effectively adjusted. Accordingly, the application of the active component must not appreciably affect the suitable pore structure of the carrier. This latter is especially to be feared if the active composition or precursor is deposited on the carrier in the form of small clustered particles. In that case the often narrow pores between the small particles of the active component lead to an additional porosity which is mostly unfavorable.
Therefore, it would be highly desirable to provide a generally suitable process for applying a catalytically active composition or precursor thereof to a carrier, particularly a shaped carrier, without appreciably changing the accessible area and the pore structure.